The use of an electronic instrument such as a spectrum analyzer to display on a cathode ray tube the amplitude and frequency of an electric signal is well known in the art. Unlike an oscilloscope which may be capable of displaying the waveform of a single transient, a spectrum analyzer usually requires a repetitive signal in order to provide a satisfactory display. This is so because at any one instant, the receiver portion of a sweeping spectrum analyzer can only be tuned to a single component of a frequency spectrum. Thus, if a frequency present in a single transient is not in coincidence with the frequency to which the spectrum analyzer is tuned at the time of occurance, it will not be displayed. (For examples, see "Spectrum Analyzer Measurements," Circuit Concepts, 1969, Tektronix, Inc., first edition).
Obviously, the capability of the spectrum analyzer to measure signals must infer that the oscillator(s) in the spectrum analyzer must be more stable than those whose frequency is to be measured.
The stabilization techniques of insuring that the oscillator(s) in the spectrum analyzer be more stable than those whose frequency is to be measured are well known in the art as step-tuned phaselock stabilization and continuously tuned phaselock stabilization. The continuously tuning, or sweeping, phaselock system is much more complicated than the step-tuned system but the stability improvement thereover makes the sweeping system the most attractive. However, each time the tuning frequency of a device under test is changed, the center frequency of the sweep generator must be manually adjusted else, the display of the electrical signal on the device may be positioned out of view. Thus, considerable attention and an appreciable amount of time by an operator are required.